1. Field of the Invention
The present invention relates to circuit boards, and more particularly, to an electrically connecting structure of a circuit board.
2. Description of the Prior Art
The widely used flip-chip technology allows a semiconductor chip to electrically connect with a circuit board. The semiconductor chip has an active surface with a plurality of electrode pads, and the circuit board is provided with electrically connecting pads corresponding in position to the electrode pads. Soldering structures are formed between the electrode pads and the electrically connecting pads to provide electrical and mechanical connections between the semiconductor chip and the circuit board.
Referring to FIGS. 1A to 1E, cross-sectional views depicting a conventional method for fabricating electrically connecting structures of a circuit board are shown. As shown in FIG. 1A, a circuit board body 10 is provided. A plurality of electrically connecting pads 11 are formed on at least one surface of the circuit board body 10. An insulating protection layer 12 is then formed on the circuit board body 10, which has openings 120 to expose the electrically connecting pads 11. As shown in FIG. 1B, a screen is disposed on the insulating protection layer 12 having openings 130 corresponding in position to the electrically connecting pads 11 for exposing the electrically connecting pads 11. Due to the alignment precision, the diameter of the openings 130 of the screen 13 are larger than those of the openings of the insulating protection layer 12. As shown in FIG. 1C, the openings 130 are coated with a soldering material 14. As shown in FIG. 1D, the screen 13 is removed to expose the soldering material 14. Since the openings of the screen 13 are larger than those of the insulating protection layer 12, wings 140 of the soldering material 14 are formed on the insulating protection layer 12. As shown in FIG. 1E, electrically conductive elements 14′ with wings 140′ are formed from the soldering material 14.
As regards a structure formed by the method above, the electrically conductive elements 14′ have wings 140′, so the electrically conductive elements 14′ cover a certain area. A definite pitch must be maintained between the electrically conductive elements 14′ to avoid short circuits therebetween when the circuit board body 10 and the semiconductor chip are electrically connected to each other. No high-density, fine-pitch electrically conductive structures can be implemented, not only because the wings 140 of the electrically conductive elements 14′ cover a certain area, but because of the irreducible pitch between the electrically conductive elements 14′.
Therefore, there is an urgent need to develop electrically conductive elements 14′ of a circuit board without wings 140′ with a view to increasing circuit layout density and achieving finer pitches.